Have you had problems with rareseeds.com?Found some websites. You can see from the pictures the difference in color, depending on soil. Some are snow white others are lemon yellow.
http://www.anniesheirloomseeds.com/great-white-tomato-organic/
http://www.totallytomato.com/dp.asp?pID=00323
http://heirloomseeds.com/great-white-tomato.html
This last one has good reviews, but I wouldn't recommend buying seeds from them:
http://www.rareseeds.com/great-white/
If you want to order but aren't sure where to get them from- when I worked on an organic farm we got all (hundreds!) of our tomatoes from Totally Tomatoes and we had extremely good germination/survival rate. Almost all the seeds sprouted and hardly any of them ever died. Totally Tomatoes is known for being reliable so that's who I'd go with personally!
Also some of the reviews compare Great White to fruity taste like pineapple or citrus?? Apparently a lot of people think so, but I didn't. I don't really know how to describe their taste, but they were dense and juicy. If you've ever tried the Cherokee Purple I mentioned above, they taste similar.